The protozoan parasites of the genus Plasmodium have an obligate sexual phase in their life cycle. Sexual stages play a pivotal role in the transmission of parasites from a vertebrate host to an insect vector. Infections in humans by the asexual stages of P. falciparu on the other hand, are responsible for hundreds of millions of clinical cases and millions of death annually. Male and female gametes must fuse in the mosquito midgut for parasite transmission to occur. It is also during the sexual reproduction in the mosquitoes that novel genotypes are produced by self and random fertilization between male and female gametes. Infectivity of parasites in the mosquitoes, therefore, requires initial differentiation of parasites in the vertebrate host to form male and female gametocytes. The molecular basis for this sexual differentiation is not known. a number of proteins expressed in the sexual stages of P. falciparum have been identified and their genes cloned but their functional role in the biology of differentiation and develop of P. falciparum remains to be elucidated. Studies are proposed to investigate genes which are differentially expressed between the asexual and sexual stages. An approach based on subtraction hybridization will be employed using stage-specific cDNA libraries. Cloned genes will be characterized with respect to encoded protein products, subcellular localization and stage-specific expression of RNA transcripts in various developmental stages of gametocytes. Finally, studies based on genetic transformation of parasites will evaluate the functional role of these genes and their encoded products in the sexual differentiation and development of P. falciparum. a molecular and biochemical understanding of the process of the sexual differentiation will suggest novel mechanisms and is likely to identify new strategies for interfering with the sexual development of P. falciparum.